Antioxidant combination of esters and amines

ABSTRACT

Ester such as 3,6,9-trioxyundecane-1,11-bis (3-n-dodecylthiopropionate) when combined with conventional free amine antioxidants such as N,N&#39;-diphenylparaphenylenediamine or with polymer bound antioxidants such as N-(4-anilinophenyl) methacrylamide produce improved antioxidant systems for SBR and NBR type polymers.

This invention relates to antioxidant systems. More particularly itrelates to butadiene/styrene (SBR) and butadiene/acrylonitrile (NBR)polymers stabilized with two component antioxidant systems.

Those skilled in the art are constantly searching for new and moreeffective antioxidant systems which are useful for the protection ofpolymers. The need for such improved systems is the result of usingpolymers under more demanding and rigorous conditions. In particular,the use of polymers in automotive applications and the like, forexample, as gaskets, has subjected polymers to high temperatures forgreat periods of time, thereby requiring long-lasting and persistentprotection.

It is an object of the present invention to provide such long-lastingand persistent antioxidant systems. It is another object of the presentinvention to provide SBR and NBR polymer compositions which are wellstabilized against oxidative degradation. Other objects will becomeapparent as the description proceeds.

The objects of the present invention are accomplished by thestabilization of SBR and NBR polymers with at least one amineantioxidant and at least one ester.

The ester has the following structural formula: ##STR1## wherein n is aninteger from 1 to 4, wherein R is selected from the group consisting ofhydrogen and methyl, wherein when n is 1, R¹ is selected from the groupconsisting of alkyl radicals having 1 to 18 carbon atoms, aryl radicalshaving 6 to 12 carbon atoms, aralkyl radicals having 7 to 12 carbonatoms and cycloalkyl radicals having 5 to 12 carbon atoms wherein when nis 2, R¹ is selected from the group consisting of alkylene radicalshaving 2 to 18 carbon atoms, cycloalkylene radicals having 5 to 12carbon atoms, arylene radicals having 6 to 12 carbon atoms, ##STR2##polyalkyl glycol ether radicals having the following structure

    -- CH.sub.2 -- CH.sub.2 -- O).sub.n 1 CH.sub.2 -- CH.sub.2 --

wherein n¹ is an integer from 1 to 7, a thioether radical having thefollowing structure

    -- CH.sub.2 -- CH.sub.2 -- S -- CH.sub.2 -- CH.sub.2 --

wherein when n is 3 or 4, R¹ is an aliphatic hydrocarbon radical havingthe formula CyH2y+2-n, wherein y is an integer from 3 to 6 and whereinR² is selected from the group consisting of alkyl radicals having 1 to24 carbon atoms, (preferably primary alkyl), aryl radicals having 6 to12 carbon atoms and aralkyl radicals having 7 to 12 carbon atoms.

The amine antioxidants are selected from the group consisting ofN,N'-di-substituted-p-phenylenediamines, substituted diphenylamines, andboth polymerized and non-polymerized derivatives of2,2,4-trimethyl-1,2-dihydroquinoline (U.S. Pat. No. 3,244,683) as wellas the amide and imide age resisters described in U.S. Pat. No.3,658,769. The N,N'-di-substituted-p-phenylenediamines have thefollowing structural formula: ##STR3## wherein R³ and R⁴ are selectedfrom the group consisting of alkyl radicals having 3 to 12 carbon atoms,aryl radicals having 6 to 12 carbon atoms and aralkyl radicals having 7to 12 carbon atoms and the diphenyl amines: ##STR4## wherein R⁵, R⁶, R⁷and R⁸ are selected from the group consisting of hydrogen, alkylradicals having 1 to 20 carbon atoms and aralkyl radicals having 7 to 12carbon atoms.

The teachings of U.S. Pat. No. 3,658,769 particularly as they relate tothe description of the imide and amide compounds and their use in thepreparation of SBR and NBR type polymers are incorporated herein byreference.

The compounds of U.S. Pat. No. 3,658,769 may be used within the practiceof the present invention in free form such as described at column 6 ofsaid reference, lines 62-67, or by polymerization techniques whereinthey are chemically made part of the polymer as described throughoutsaid reference, for example, at column 4, lines 58-62. The bestsynergistic results within the practice of the present invention areobtained when these compounds are polymerized into the polymer.

Compounds illustrating the above amines but not acting as limitationsthereof are as follows:

N,n'-diphenyl-p-phenylenediamine

N,n'-di-beta-naphthyl-p-phenylenediamine

N-o-tolyl-N'-phenyl-p-phenylenediamine

N,n'-di-p-tolyl-p-phenylenediamine

N-1,3-dimethylbutyl-N'-phenyl-p-phenylenediamine

N-1,4-dimethylpentyl-N'-phenyl-p-phenylenediamine

N-isopropyl-N'-phenyl-p-phenylenediamine

N-1-methylpropyl-N'-phenyl-p-phenylenediamine

N-cyclohexyl-N'-phenyl-p-phenylenediamine

N,n'-bis-(1-ethyl-3-methylpentyl)-p-phenylenediamine

N,n'-bis-(1,4-dimethylpentyl)-p-phenylenediamine

N,n'-bis-(1-methylpropyl)-p-phenylenediamine

4,4'-bis-(di-alpha-methylbenzyl)-diphenylamine

4,4'-dioctyldiphenylamine

4,4'-bis-(di-alpha-methylbenzyl)-diphenylamine -dinonyldiphenylamine

polymerized-2,2,4-trimethyl-1,2-dihydroquinoline

6-dodecyl-1,2-dihydro-2,2,4-trimethylquinoline

6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline

N-(4-anilinophenyl)methacrylamide

as well as the compounds listed in column 3 of U.S. Pat. No. 3,658,769.

The amine compounds are well known in the art being described along withmethods of preparation in U.S. Pat. Nos. 3,244,683; 3,658,769;3,305,584; 3,739,026; 3,781,361; 3,452,056.

The esters of the present invention are illustrated by the followingcompounds.

* 3,6,9-trioxaundecane-1,11-bis(3-n-dodecylthiopropionate)

* 3,6-dioxaoctane 1,8-bis(3-n-dodecylthiopropionate)

3,6,9-trioxaundecane-1,11-bis(3-n-dodecylthio-2-methylpropionate)

3-oxapentane-1,5-bis(3-n-dodecylthiopropionate)

The asterisk (*) compounds are preferred compounds.

Other esters illustrating the esters of the present invention are asfollows.

phenyl-(3-phenylthiopropionate)

phenyl-1,4-bis(3-t-dodecylthiopropionate)

naphthyl-1-(3-n-dodecylthiopropionate)

naphthyl-2-(3-n-octyl-2-methylpropionate)

naphthyl-1,4-bis(3-n-hexylthiopropionate)

phenyl-(3-n-dodecylthio-2-methylpropionate)

benzyl-(3-t-dodecylthiopropionate)

benzyl-(3-n-dodecylthio-2-methylpropionate)

p-xyxyl-alpha, alpha'-bis(3-n-octyl-2-methylpropionate)

o-xyxyl-alpha, alpha'-bis(3-n-dodecylthiopropionate)

ethane-1,2-bis(3-n-dodecylthiopropionate)

ethane-1,2-bis(3-t-dodecylthiopropionate)

butane-1,4-bis(3-benzylthiopropionate)

pentane-1,5-bis(3-n-hexylthio-2-methylpropionate)

propane-1,2-bis(3-n-dodecylthiopropionate)

octane-1,8-bis(3-n-tetracosylthiopropionate)

3,6,9-trioxaundecane-1,11-bis(3-phenylthiopropionate)

3,6,9-trioxaundecane-1,11-bis(3-benzylthio-2-methylpropionate)

3-oxapentane-1,5-bis(3-benzylthio-2-methylpropionate)

3-thiapentane-1,5-bis(n-octylthio-2-methylpropionate)

3-thiapentane-1,5-bis(benzylthio-2-methylpropionate)

1,1,1-trimethanolpropane-tris(3-n-octylthiopropionate)

1,1,1-trimethanolpropane-bis(3-t-dodecylthiopropionate)

pentaerythritol-tetrakis(3-phenylthiopropionate)

pentaerythritol-tetrakis(3-n-dodecylthio-2-methylpropionate)

The esters are also illustrated in U.S. Pat. Nos. 3,629,194 and3,758,549, the contents of which particularly as they relate to thedescription of the esters and their preparation, are incorporated hereinby reference.

The esters of the present invention can be prepared by reacting asuitable thiol with an ester of acrylic or methacrylic acid in thepresence of a basic catalyst such as KOH or benzyl trimethyl ammoniumhydroxide. In the case of aromatic hydroxy compounds such a process ispreferred. The esters of the present invention may also be convenientlyprepared by reacting a suitable acid with an alcohol in a known simpleacid catalyzed esterification procedure as described in U.S. Pat. No.2,601,063.

An alternate preparation of the esters involves the initial reaction ofa suitable thiol with a lower alkyl ester of acrylic or methacrylicacid. The alkylthiopropionate ester is then transesterified with a highmolecular weight glycol.

None of the compounds of the present invention are limited to the methodof their preparation.

The SBR polymers which benefit by the practice of the present inventionare those rubbery copolymers of butadiene/styrene in which the boundbutadiene content is at least 50 percent by weight.

The NBR polymers which benefit by the practice of the present inventionare those rubbery copolymers of butadiene and acrylonitrile in which thebound butadiene content is at least 55 percent by weight.

As noted earlier herein, the amide and imide compounds of U.S. Pat. No.3,658,769 when used in the practice of the present invention may bechemically as opposed to physically incorporated into the SBR and NBRpolymers by adding them to the monomer systems, i.e., butadiene andstyrene or butadiene and acrylonitrile monomers prior to polymerization.Such techniques are described in U.S. Pat. No. 3,658,769.

The level of amine compound is from 0.25 to 5.0 parts by weight per 100parts by weight of polymer. Preferably, the amine level is from 0.5 to2.0 parts by weight. The weight of ester is from 1.0 to 20 parts byweight.

The sulfur vulcanized, both compounded and uncompounded versions of theSBR and NBR polymers will benefit by the use of the antioxidantcombination. Vulcanizing agents include free sulfur or compounds whichare sulfur donors such as 2-(morpholinodithio) benzothiazole. Therubbers can be compounded with any conventional compounding ingredientssuch as carbon blacks, silica, zinc oxide, etc. The polymers areparticularly benefited when exposed to high temperature, heat-aged typeconditions. Vulcanization systems of both high and low efficiencybenefit by the practice of the present invention.

The following examples are intended to illustrate but not to limit thepractice of the present invention. Unless indicated otherwise, all partsare parts by weight per 100 parts by weight of rubber.

                  TABLE I.                                                        ______________________________________                                        Ingredients                Parts                                              ______________________________________                                        NBR.sup.(1)                100                                                Stearic acid               1                                                  Zinc oxide                 5                                                  Magnesium oxide            5                                                  Silica                     50                                                 Coupling agent             .05                                                Polyethylene glycols       2                                                  Sulfur                     .3                                                 2-(morpholinodithio)-benzothiazole                                                                       2.5                                                2-(morpholinothio)-benzothiazole                                                                         1.3                                                Zinc dimethyldithiocarbamate                                                                             0.6                                                ______________________________________                                         .sup.(1) 67/33/1.6 1,3-butadiene/acrylonitrile N-(4-anilinophenyl)            methacrylamide                                                           

The above formulation was compounded with various esters vulcanized andaged. The esters were used at the 2.5 part level. Tensile and elongationwere measured on these vulcanizates as well as on a vulcanizate of theabove formulation absent an ester, both before and after aging. Thepercent tensile and elongation retained after aging was calculated alongwith the percent tensile retained times percent elongation retainedproduct. The higher these values, the better is the resistance of thepolymer to aging.

The esters will be identified A,B,C, and D as follows:

A. 3,6-dioxaoctane-1,8-bis (3-n-dodecylthio-2-methylpropionate)

B. ethane-1,2-bis(3-n-dodecylthio-2-methylpropionate)

C. hexane-1,6-bis(3-n-dodecylthio propionate)

D. 3,6,9-trioxaundecane-1,11-bis(3-n-dodecylthiopropionate)

                  TABLE II                                                        ______________________________________                                        Air Oven Aged at 300° F.                                               % Retained                                                                    Tensile       Elongation   Tensile × Elongation                         Ester 70 hr  120 hr   70 hr 120 hr 70 hr 120 hr                               ______________________________________                                        --    64     38       60    20     3840   760                                 A     72     52       68    26     4896  1352                                 B     77     42       62    20     4774   840                                 C     75     60       73    32     5475  1920                                 D     75     57       76    33     5700  1881                                 ______________________________________                                    

In each case the tensile x elongation product was superior to thecontrol containing no ester.

In Tables III and IV are listed data on vulcanizates where the esterswere added to an NBR latex prior to compounding. The NBR was preparedfrom a monomer system of 67/33/1.61,3-butadiene/acrylonitrile/n(4-anilinophenyl) methacrylamide.

                  Table III                                                       ______________________________________                                        Air Oven Aged 70 Hours at 300° F.                                      % Retained                                                                    Ester       Tensile    Elongation   TXE                                       ______________________________________                                        --          64         34           2176                                      A           69         35           2415                                      B           67         32           2144                                      C           69         37           2553                                      D           68         37           2516                                      ______________________________________                                    

All but B possessed super TXE products. The rubber compositions werealso aged in ASTM oil #3 for 70 hours at 300° F. followed by aging for70 hours in an air oven at 300° F. The results are listed in Table IV.

                  Table IV                                                        ______________________________________                                        % Retained                                                                    Ester       Tensile    Elongation   TXE                                       ______________________________________                                        --          48         24           1152                                      A           63         38           2394                                      B           57         26           1482                                      C           65         39           2535                                      D           70         47           3290                                      ______________________________________                                    

In each case the esters improved the resistance to aging.

A butadiene/acrylonitrile latex was compounded with three differentamine antioxidants alone (2 parts) and in combination (2 parts) with 8parts of ester D. The latex was coagulated, compounded, vulcanized andaged as the compositions in Table IV. The tensile and elongation resultsare listed in Table V.

                  TABLE V                                                         ______________________________________                                        % Retained                                                                    Amine     Ester     Tensile   Elongation                                                                            TXE                                     ______________________________________                                        X.sup.(1) --        35        22       770                                    X         D         64        59      3776                                    Y.sup.(2) --        56        43      2408                                    Y         D         63        73      4599                                    Z.sup.(3) --        26        16       416                                    Z         D         44        53      2332                                    ______________________________________                                    

In each case the ester greatly improved the TXE product.

(1) Polymerized 2,2,4-trimethyl-1,2-dihydroquinoline

(2) Mixture of diaryl paraphenylene diamines

(3) 4,4'-bis(di-alpha-methylbenzyl)-diphenylamine

An SBR (butadiene/styrene rubber) containing 0.75 part of a phenolicantioxidant was aged with no additional antioxidant present, with amineantioxidant Z alone (1 part), with ester D alone (1 part) and with Z(0.75 part) and D (0.25 part) in combination.

The aged data (air oven aged 70 hours at 121° C.) is listed in Table VI.

                  TABLE VI                                                        ______________________________________                                        % Retained                                                                    Z        D        Tensile   Elongation                                                                              TXE                                     ______________________________________                                        1.0      --       58        35        2030                                    --       1.0      43        28        1204                                    0.75     0.25     75        45        3375                                    --       --       62        35        2160                                    ______________________________________                                    

As shown above, the combination aged much better than the individualcomponents.

Another SBR polymer containing 0.75 part of amine antioxidant Y wasstabilized with additional amounts of other amines alone (1 part) and incombination (0.75 part) with ester D (0.25 part). Aged comparisons weremade between vulcanizates so compounded as well as the vulcanizates withester D alone (1 part) added and no additional stabilizer.

The vulcanizates were aged 70 hours at 121° C. The results are listed inTable VII.

                  TABLE VII                                                       ______________________________________                                        % Retained                                                                    Amine       Ester     Tensile  Elongation                                                                             TXE                                   ______________________________________                                        --          D         66       45       2970                                  Z           --        72       39       2808                                  Z           D         77       53       4081                                  SDPA.sup.(4)                                                                              --        65       42       2730                                  SDPA        D         88       59       5192                                  NAPMA.sup.(5)                                                                             --        51       31       1581                                  NAPMA       D         85       55       4675                                  --          --        58       36       2090                                  ______________________________________                                         .sup.(4) Reaction product of styrene and diphenylamine contains               4,4'-bis(alphamethylbenzyl)-diphenylamine                                     .sup.(5) N-(4-anilinophenyl)methacrylamide                               

The combinations were superior to the individual components even thoughthe total stabilizer system was maintained at the 1.0 part level.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

I claim:
 1. A synergistic antioxidant system comprising an ester havingthe following structural formula ##STR5## and an amine antioxidantselected from the group consisting of polymerized and non-polymerizedderivatives of 2,2,4-trimethyl-1,2-dihydroquinoline,N,N'-di-substituted-p-phenylenediamines having the following structuralformula ##STR6## diphenyl amines having the following structural formula##STR7## and amides having the following structural formula ##STR8##wherein n is an integer from 1 to 4, wherein R is selected from thegroup consisting of hydrogen and methyl, wherein when n is 1, R¹ isselected from the group consisting of alkyl radicals having 1 to 18carbon atoms, aryl radicals having 6 to 12 carbon atoms, aralkylradicals having 7 to 12 carbon atoms and cycloalkyl radicals having 5 to12 carbon atoms wherein when n is 2, R¹ is selected from the groupconsisting of alkylene radicals having 2 to 18 carbon atoms,cycloalkylene radicals having 5 to 12 carbon atoms, arylene radicalshaving 6 to 12 carbon atoms, ##STR9## polyalkyl glycol ether radicalshaving the following structure

    -- CH.sub.2 -- CH.sub.2 -- O).sub.n 1 CH.sub.2 -- CH.sub.2 --

wherein n¹ is an integer from 1 to 7, a thioether radical having thefollowing structure

    -- CH.sub.2 -- CH.sub.2 -- S -- CH.sub.2 -- CH.sub.2 --

wherein when n is 3 or 4, R¹ is an aliphatic hydrocarbon radical havingthe formula CyH2y+2-n, wherein y is an integer from 3 to 6 and whereinR² is selected from the group consisting of alkyl radicals having 1 to24 carbon atoms, aryl radicals having 6 to 12 carbon atoms and aralkylradicals having 7 to 12 carbon atoms, wherein R³ and R⁴ are selectedfrom the group consisting of alkyl radicals having 3 to 12 carbon atoms,aryl radicals having 6 to 12 carbon atoms and aralkyl radicals having 7to 12 carbon atoms, wherein R⁵, R⁶, R⁷ and R⁸ are selected from thegroup consisting of hydrogen, alkyl radicals having 1 to 20 carbon atomsand aralkyl radicals having 7 to 12 carbon atoms, wherein R¹² is anarylene radical having 6 to 12 carbon atoms, R⁹ and R¹⁰ are selectedfrom the group consisting of hydrogen, alkyl radicals having 1 to 4carbon atoms and alkoxy radicals having 1 to 4 carbon atoms, R11 isselected from the group consisting of hydrogen, alkyl radicals having 1to 4 carbon atoms and alkoxy radicals having 1 to 4 carbon atoms andwherein R¹³ and R¹⁴ are selected from the group consisting of hydrogenand alkyl radicals having 1 to 4 carbon atoms and R¹⁵ is selected fromthe group consisting of hydrogen, alkyl radicals having from 1 to 4carbon atoms and aryl radicals having 6 to 12 carbon atoms.
 2. A rubberypolymer containing the antioxidant system of claim 1 wherein the rubberypolymer is selected from the group consisting of copolymers of butadieneand styrene wherein the bound butadiene content is at least 50 percentby weight and copolymers of butadiene and acrylonitrile wherein thebound butadiene content is at least 55 percent by weight and wherein theamine antioxidant is present in the amount 0.25 to 5.0 parts and theester is present in the amount of 1.0 to 20 parts all parts being byweight per 100 parts by weight of rubbery polymer.
 3. The rubberypolymer of claim 2 wherein the amine antioxidant is an amide accordingto structural formula IV and is chemically incorporated into the rubberypolymer in the form of a segmeric unit having the following structure##STR10##
 4. The rubbery polymer according to claim 3 wherein the esteris 3,6,9-trioxaundecane-1,11-bis(3-n-dodecylthiopropionate).